The present invention relates to a light beam scanner for use in image forming apparatus such as a printer, facsimile apparatus and a copier. The invention also relates to an image forming apparatus provided with such a light beam scanner.
An image forming apparatus, such as a copier or a printer, includes a light beam scanner configured to scan the surface of an image carrier which is uniformly electrostatically charged in advance by static charger means with image-forming light, for example, laser beam modulated in accordance with signals based on image data.
Light beam scanners of the laser beam type adaptable to an increase in scanning speed are frequently used in such image forming apparatus because such a scanner exhibits a high modulating rate in modulating light into image-forming light in accordance with image data.
Conventionally, an image forming apparatus for color image formation has been provided with individual light beam scanners for respective elementary colors of a color image. However, the provision of plural light beam scanners in an image forming apparatus does not meet the demand for reduction in apparatus size and price.
In attempt to overcome this inconvenience, a light beam scanner has been developed having a configuration wherein a single polygonal mirror deflects plural light beams emitted from light sources provided for respective elementary colors in the primary scanning direction at constant angular velocity and the light beams thus deflected are further deflected at a constant velocity by a single transformation means and then separated from each other toward respective elementary color image carriers, as disclosed in Japanese Patent Application Laid-Open Nos. S63-009361 A and 2000-347116 for example.
In deflecting plural light beams at constant angular velocity at a time by causing the plural light beams to become incident on the same surface of the polygonal mirror, further deflecting the plural light beams at a constant velocity by the transformation means and then separating the light beams from each other by reflecting members, it is required that the plural light beams become incident on the reflective surface of the polygonal mirror at different incident angles in the secondary scanning direction.
However, if a light beam is caused to become incident on a reflective surface of the polygonal mirror at an angle in the secondary scanning direction, a scanning line formed by the light beam deflected with rotation of the polygonal mirror becomes curved. This phenomenon is generally called “bow”. The curvature of such a bow increases with increasing incident angle of a light beam on the polygonal mirror in the secondary scanning direction. Light beams incident on the polygonal mirror from opposite sides of a primary optical axis plane will form respective scanning lines curved in different directions after deflection by the polygonal mirror. The primary optical axis plane is perpendicular to a reflective surface of the polygonal mirror and, hence, light passing in the primary optical axis plane becomes incident on the reflective surface at an incident angle of 0°. Accordingly, light beams incident on the reflective surface of the polygonal mirror from opposite sides of the primary optical axis plane form scanning lines curved in opposite directions. Thus, the resulting scanning lines deviate largely from each other, which will cause a color drift to occur in a color image forming apparatus. Such a color drift raises a problem of considerably degraded image quality.
A feature of the present invention is to provide a light beam scanner capable of decreasing an error in the secondary scanning direction in forming scanning lines with plural light beams. Another feature of the present invention is to provide an image forming apparatus provided with such a light beam scanner.